This invention relates to a surface treatment process for titanium, and in a particular aspect, to a process for treating the surface of titanium structural members preparatory to the application thereto of adhesives, sealants, organic coatings and the like.
With the accelerating usage of titanium alloys for aircraft structures, the problems of promoting adhesion of coatings (e.g., sealants, adhesives and paints) to this metal have become increasingly apparent. In general, adhesion of organic materials to titanium has been rather poor. Adhesion can be improved by surface treatment techniques in which anodic or chemical films are deposited on the titanium. During the past decade, a host of titanium surface treatments have been reported, most of which have been developed for prevention of galling and fretting. The "Pasa-Jell" conversion coating process (developed by Semco Sales and Service Co., Los Angeles, Calif.) and the phosphate-fluoride conversion coating process (U.S. Pat. No. 2,864,732) have been widely used for surface treatment prior to adhesive bonding of titanium and titanium-containing alloys. When titanium adherends surface treated by either of these methods are subjected to adhesive bonding, initial bond strengths are usually acceptable. However, when the bonds are exposed to high humidity environments, the quality of the bonds is seriously deteriorated. As a consequence, numerous bond-joint failures in titanium adhesive sandwich structures have been experienced in commercial aircraft. Most of the titanium adherends in these structures were treated by the phosphate-fluoride process prior to adhesive bonding. Service failure analyses have shown that the failures invariably occur at the adhesive-metal interface while the cured adhesive remains generally unchanged and exhibiits good cohesive bonding. Hence, it appears that in service environments, the adhesive-metal interface is the weakest point in adhesively bonded structures.
It is therefore an object of this invention to overcome or mitigate the foregoing and other shortcomings of the prior art by providing a process for producing on titanium and titanium alloys coatings that are uniform and strong and will form environmentally stable bonds to adhesives and other coatings. It is another object of this invention to provide titanium structural members having prepared surfaces that will exhibit superior adhesive bonding and adherence to sealants and coatings. A further object is to provide adhesively bonded titanium composites exhibiting excellent structural integrity in high humidity environments. Other objects and advantages of this invention will be apparent from the following.